14.4 2017 ASHRAE Handbook—Fundamentals

Table 1 Design Conditions for Atlanta, GA, USA (see Table 1A for Nomenclature)

Climatic Design Information 14.5

over the period 2005 to 2014. Pressure is estimated from station’selevation.

Aerosol turbidity data (in the form of separate evaluations ofaerosol optical depth and Ångström exponent) received specialattention, because they are the primary inputs that affect the accu-racy of direct and diffuse irradiance predictions under clear skies.Spaceborne retrievals of aerosol optical depth at various wave-lengths from NASA’s Multi-angle Imaging SpectroRadiometer(MISR; www-misr.jpl.nasa.gov) and two Moderate ResolutionImaging Spectroradiometer (MODIS; modis-atmos.gsfc.nasa.gov)instruments were used between 2000 and 2014 and compared to ref-erence data from a large number of ground-based sites, mostly fromthe Aerosol Robotic Network (AERONET; aeronet.gsfc.nasa.gov),after appropriate scale-height corrections to remove artifacts fromthe effect of elevation (Gueymard and Thevenard 2009). Regionalcorrections of the satellite data were devised to remove as much bias

as possible, compared to the reference ground-based data. To fillmissing data or correct biased satellite observations, modeled aero-sol datasets were used, including 10 years (2003 to 2012) of simu-lated monthly-average aerosol optical depth from the MonitoringAtmospheric Composition and Climate (MACC) reanalysis model(Eskes et al. 2015; Inness et al. 2013) and 13 years (2002 to 2014)of MERRA-2 reanalysis data (Molod et al. 2015). Results from theREST2 model (Gueymard 2008) were then fitted to the simple two-parameter model described in this chapter. The fits enable a conciseformulation requiring tabulation, on a monthly basis, of only twoparameters per station, referred to here as the clear-sky beam anddiffuse optical depths. Details about the fitting procedure can befound in Thevenard and Gueymard (2013).

Global horizontal irradiance at the surface, and its standard devi-ation, were calculated from the Clouds and the Earth’s RadiantEnergy System (CERES) Energy Balanced and Filled (EBAF) data-set (ceres.larc.nasa.gov/products.php?product=EBAF-Surface). Fromthe available 1°×1° dataset, a bilinear interpolation, without altitudeadjustment, was made given the station latitude and longitude forthe period 2000 to 2014.

Calculation of Design ConditionsValues of ambient dry-bulb, dew-point, and wet-bulb temperature

and wind speed corresponding to the various annual percentiles repre-sent the value that is exceeded on average by the indicated percentageof the total number of hours in a year (8760). The 0.4, 1.0, 2.0, and5.0% values are exceeded on average 35, 88, 175, and 438 h per year,respectively, for the period of record. The design values occur morefrequently than the corresponding nominal percentile in some yearsand less frequently in others. The 99.0 and 99.6% (cold-season) valuesare defined in the same way but are usually viewed as the values forwhich the corresponding weather element is less than the design con-dition for 88 and 35 h, respectively.

Simple design conditions were obtained by binning hourly datainto frequency tables, then deriving from the binned data the designcondition having the probability of being exceeded a certain per-centage of the time. Mean coincident values were obtained bydouble-binning the hourly data into joint frequency matrices, thencalculating the mean coincident value corresponding to the simpledesign condition.

Coincident temperature ranges were also obtained by double-binning daily temperature ranges (daily maximum minus mini-mum) versus maximum daily temperature. The mean coincidentdaily range was then calculated by averaging all bins above the sim-ple design condition of interest.

The weather data sets used for the calculations often containmissing values (either isolated records, or because some stationsreport data only every third hour). Gaps up to 6 h were filled by lin-ear interpolation to provide as complete a time series as possible.Dry-bulb temperature, dew-point temperature, station pressure, andhumidity ratio were interpolated. However, wind speed and direc-tion were not interpolated because of their more stochastic andunpredictable nature.

Some stations in the ISD data set also provide data that were notrecorded at the beginning of the hour. When data at the exact hourwere missing, they were replaced by data up to 0.5 h before or after,when available.

Finally, psychrometric quantities such as wet-bulb temperatureor enthalpy are not contained in the weather data sets. They werecalculated from dry-bulb temperature, dew-point temperature, andstation pressure using the psychrometric equations in Chapter 1.

Measures were taken to ensure that the number and distribution ofmissing data, both by month and by hour of the day, did not introducesignificant biases into the analysis. Annual cumulative frequencydistributions were constructed from the relative frequency distribu-tions compiled for each month. Each individual month’s data were

Table 1A Nomenclature for Tables of Climatic Design Conditions

CDDn Cooling degree-days base n°F, °F-dayCDHn Cooling degree-hours base n°F, °F-hourDB Dry-bulb temperature, °FDBAvg Average daily dry-bulb temperature, °FDBSD Standard deviation of average daily dry-bulb

temperature, °FDP Dew-point temperature, °FEbn,noon Clear-sky beam normal irradiances at solar noon,

Btu/h·ft2

Edh,noon Clear-sky diffuse horizontal irradiance at solar noon, Btu/h·ft2

Elev Elevation, ftEnth Enthalpy, Btu/lb base 0°F and 1 atm pressureHDDn Heating degree-days base n°F, °F-dayHR Humidity ratio, grmoisture/lbdry airLat Latitude, °NLong Longitude, °EMCDB Mean coincident dry-bulb temperature, °FMCDBR Mean coincident dry-bulb temp. range, °FMCWB Mean coincident wet-bulb temperature, °FMCWBR Mean coincident wet-bulb temp. range, °FMCWS Mean coincident wind speed, mphMDBR Mean dry-bulb temp. range, °FPCWD Prevailing coincident wind direction, °

(0 = North; 90 = East)Period Years used to calculate the design conditionsPrecAvg Average precipitation, in.PrecMax Maximum precipitation, in.PrecMin Minimum precipitation, in.PrecStd Standard deviation of precipitation, in.RadAvg Monthly mean daily all-sky radiation, Btu/ft2·dayRadStd Standard deviation of monthly mean daily radiation,

Btu/ft2·dayStdP Standard pressure at station elevation, psitaub Clear-sky optical depth for beam irradiancetaud Clear-sky optical depth for diffuse irradianceTime Zone Hours ahead or behind UTC, and time zone codeWB Wet-bulb temperature, °FWBAN Weather Bureau Army Navy numberWMO# Station identifier from the World Meteorological

OrganizationWS Wind speed, mphWSAvg Monthly average wind speed, mphNote: Numbers (1) to (45) and letters (a) to (p) are row and column referencesto quickly point to an element in the table. For example, the 5% design wet-bulb temperature for July can be found in row (31), column (k).